Air induction valve for exhaust emission control system

ABSTRACT

An air induction valve for use in the air injection unit of an exhaust emission control system for an internal combustion engine in which exhaust pressure pulsations are used to induce air flow through the air injection unit, the air induction valve including check valves to permit the ingress of air into the air injection unit while preventing egress of exhaust gases therefrom and a diaphragm actuated control valve to prevent air injection during engine deceleration.

United States Patent [191 Ranit et al..

AIR INDUCTION VALVE FOR EXHAUST EMISSION CONTROL SYSTEM Inventors: ErnstL. Ranft, Webster, N.Y.;

Kenneth R. Pfrengle, Atlanta, Ga.

General Motors Corporation, Detroit, Mich.

Mar. 1, 1971 (Under Rule 47) Appl. No.: 119,508

Assignee:

Filed:

US. Cl 60/290, 60/293, 60/308 Int. Cl F02b 75/10 Field of Search 60/289,290, 293,

References Cited UNITED STATES PATENTS Hyde 60/290 Au. M, 1973 3,543,51012/1970 Kauffmann 60/290 3,653,2l2 4/1972 Gast 60/293 3,662,541 5/1972Sawada 60/293 Primary Examiner-Douglas Hart Attorney-Jean L. Carpenterand Arthur N. Krein [5 7] ABSTRACT An air induction valve for use in theair injection unit of an exhaust emission control system for an internalcombustion engine in which exhaust pressure pulsations are used toinduce air flow through the air injection unit, the air induction valveincluding check valves to permit the ingress of air into the airinjection unit while preventing egress of exhaust gases therefrom and adiaphragm actuated control valve to prevent air injection during enginedeceleration.

8 Claims, 5 Drawing Figures -Patented Aug. 14, 19 73 3,751,915

I N VEN'TORS Y mp /A mfi /ea TORNE Y AIR INDUCTION VALVE FOR EXHAUSTEMISSION CONTROL SYSTEM This invention relates to an emission controldevice and, specifically, to an air induction valve for use on the airinjection unit of an exhaust emission control system of an internalcombustion engine in which exhaust system pressure pulsations are usedto induce air flow to the exhaust ports of the engine to deliver air tothe stream of exhaust gases as they are emitted from the enginecombustion chambers.

During recent years, increasing emphasis has been placed on reducing theamount of unburned constituents, such as hydrocarbon and carbonmonoxide, present in the exhaust gases emitted from internal combustionengines. One of the most effective arrangements devised to accomplishthis reduction is the air injection system. In this system, an enginedriven air pump delivers air to the stream of hot exhaust gases as theyare emitted from the engine combustion chambers. Utilizing the heat ofthe exhaust gases, the injected air supports additional burning ofexhaust gases in the engine exhaust passages to thereby reduce theamount of unburned constituents in the exhaust gases discharged to theatmosphere.

It has now been found that sufficient air induction can be obtainedwithout the use of an engine driven air pump by utilizing a suitablytuned exhaust system to obtain maximum air flow induced by exhaustpressure pulsations in the exhaust system of the engine. Such a systemis described in detail in copending US. application Ser. No. 85,379filed on Oct. 30, 1970 now US. Pat. No.'3,653,2l2, in the names ofRichard A. Cast and Harry R. Mitchell. In this type of exhaust emissioncontrol system, an internal combustion engine having an N number ofcombustion chambers is provided with one or more exhaust conduits, eachincluding an exhaust manifold and an exhaust pipe, and each of theexhaust conduits being connected to a maximum of four or N/Z number ofcombustion chambers. Each exhaust manifold is provided with an airinduction unit having passages extending into the exhaust manifold todirect air toward the combustion chambers exhausting into that exhaustmanifold, the passages being connected by an air induction valve to theatmosphere. Each of the exhaust conduits is of a predetermined lengthfor a particular range of engine operating speeds to effect maximum flowof air into the exhaust system.

It is, therefore, a principal object of this invention to provide animproved exhaust emission control system having an air induction valveadapted to prevent backfiring of the engine during engine decelerationand to prevent the egress of exhaust gases from the air induction unitof this system to the atmosphere.

Another object of this invention is to provide an improved exhaustemission control system having an air induction valve provided with aplurality of check valves to control the flow of induced air whilepreventing backflow of exhaust gases, the check valves being providedwith thermostatic stops which force the check valves to a closedposition during high operating temperature conditions which conditionswould occur during reverse flow of exhaust gases.

These and other objects of the invention are obtained by means of anexhaust emission control system for an internal combustion engine havingan induction passage and N number of combustion chambers wherein thesystem comprises exhaust conduits each including an exhaust manifold andan exhaust pipe with the exhaust conduit preferably being connected tonot more than N/Z combustion chambers in the case of a V-8 engine, forexample, and an air induction unit having passages to deliver air to thestream of exhaust gases as they are emitted from the combustionchambers, and an air induction valve to control the flow of air into theair induction system. The air induction valve consists of a housingdefining a passage through which air can flow and including an inlet incommunication with the atmosphere and a discharge connected to the airinduction unit. Check valves, provided in the passage, are responsive tosub-ambient exhaust pressure pulsations to control the flow of airthrough the air induction unit to the combustion chambers whilepreventing the backflow of exhaust gases. A pressure responsivediaphragm actuated valve is used to control the flow of air to the checkvalves and is responsive to pressure changes in the induction passage tothe engine, a timing valve being provided to effect a controlequalization of pressure on opposite sides of the diaphragm.Thermostatic stops are provided for each of the check valves and arepositioned so that under high temperature conditions, which conditionswill occur as the exhaust gases flow back toward the check valves, thethermostatic stops are forced against the check valves pushing the sameagainst their seats to prevent the discharge of exhaust gases throughthe air induction valve.

For a better understanding of the invention, as well as other objectsand further features thereof, reference is had to the following detaileddescription of the invention to be read in connection with theaccompanying drawings, wherein:

FIG. I is a perspective view of a V-8 internal combustion engine havingan exhaust emission control assembly with an air induction valveconstructed in accordance with the invention;

FIG. 2 is an enlarged top view of the air induction valve of FIG. 11,with parts broken away to show details of its structure and showing thereed valves closed by thermostatic stops;

FIG. 3 is a sectional view taken along line 3-3 of FIG. 2;

FIG. 41 is a sectional view of the air induction valve of FIG. 2 showingthe thermostatic stops in position to allow normal operation of the reedvalves; and,

FIG. 5 is an enlarged view of a portion of FIG. 4 showing the details ofthe timing valve.

Referring now to FIG. ll, there is illustrated an internal combustionengine MD, shown as a V-8 engine for purposes of illustration only,which is provided with a carburetor Ill and an air cleaner I2 thereonmounted to supply an air-fuel mixture to the induction passages, notshown, of the engine. Each bank of cylinders of the engine is providedwith an exhaust manifold I3 connected to an exhaust pipe 14 Each of theexhaust manifolds has an air induction unit in the form of an airmanifold 15 positioned adjacent to it which is provided with a series ofpassages in injection tubes llfi extending into the exhaust manifold andterminating downstream of and closely adjacent to the exhaust valve ineach of the cylinders, not shown, whereby air delivered through theseinjection tubes is injected into the stream of exhaust gases adjacent tothe combustion chamber exhaust valve.

Each of the air manifolds is connected by a suitable conduit, such asconduit coupling 117 to the outlet of an air induction valve, generallydesignated 30, constructed in accordance with the invention, which issuitably supported on the engine 10. Clean air is supplied to the airinduction valve 30 by means of an air inlet hose 18, which in thearrangement shown, for purpose of illustration only, has its inletconnected to the air cleaner l2 downstream of the filter element, notshown, therein. For a purpose to be described, the air induction valve30 is also suitably connected to the induction passage of the engine, asfor example, by being connected to the air-fuel intake manifold 19 ofthe engine by a hose conduit 21.

In the exhaust emission control system shown in FIG. I, exhaust systempressure pulsations in the exhaust system of the engine are used toinduce air into the exhaust ports of the engine via the air injectiontubes previously described. High speed check valves are used in the airinduction valve 30 to allow air to flow into the exhaust port when theexhaust pressure in the exhaust system fluctuates below atmosphericpressure. These sub ambient depressions occur because the pressure wavesproduced by combustion chamber flowdown reflect from the open end of anexhaust pipe as an expansion wave. Pressure waves from all cylindersconnected to a common exhaust pipe combines to form a composite wave. Toinduce sufficient air for emission control, as required by current andproposed emission control standards, the exhaust system must be tailoredto produce positive pulsation frequencies which the check valve of theair induction valve can follow effectively.

In order to produce maximum air flow over a suitably wide range ofengine speeds, it is desirable that not more than four combustionchambers exhaust into an exhaust conduit and that each exhaust conduitbe maintained isolated from any other exhaust conduit for apredetermined distance to effect tuning of this exhaust conduit. Afterthis predetermined length for each exhaust conduit, they can beconnected in a suitable manner for exhaust to a common muffler, notshown if so desired.

Thus, in FIG. 1, both the right and left-hand banks of the engine wouldbe provided with left and right-hand exhaust manifolds, the right-handexhaust manifold only being shown, and right and left-hand exhaust pipes14 and 14a, respectively, the length of each exhaust conduit thus formedby an exhaust manifold and exhaust pipe would be of a preferred length,as disclosed in the aforementioned Gast et al US. application Ser. No.85,379, after which they could be connected by a suitable Y-connection22 to a common muffler and exhaust pipe, not shown. It should beappreciated that in the case of a six cylinder engine, not shown, if theengine is a V-6, each bank of the cylinders could be isolated as in theV-8 engine of FIG. 1, whereas if it is an in-line six cylinder engine,exhaust manifolding could be made on a 3-3 basis or a 4-2 basis, the 3-3cylinder arrangement being preferred since it will provide higherinduced air flow rates over a wider speed range than the 42 cylinderarrangement.

Referring now to FIGS. 2, 3, 4 and 5, there is shown an air inductionvalve 30 adapted to supply air to opposite banks of the V-8 engine ofFIG. I. In the operation of the air induction valve 30, atmospheric airflows through air inlet hose 18 through flanged inlet duct 34 of thevalve casing past a normally open control valve 71 into inlet chamber38. From the chamber 38, the air flows through a plurality of reed typecheck valves 51 which are adapted to open as the exhaust manifoldpressure drops below atmospheric to permit air flow through the outlet42 of reed valve covers 32 for discharge through the air induction unitinto the exhaust system.

In order to prevent backfire, the control valve 71 is connected foractuation by a pressure responsive diaphragm which will close thecontrol valve 71 to prevent air injection during engine deceleration.Intake manifold vacuum is supplied to the upper side of the diaphragm sothat during deceleration, the high induction vacuum will, in effect,pull the diaphragm upward to close the control valve 71. A timing valveis used to gradually effect a balance of the pressure on the oppositeside of the diaphragm so that the control valve H is closed only for apredetermined interval.

As illustrated, the air induction valve 30 consists essentially of amultiple piece housing having a central or main body 31, opposeddischarge casings or reed valve covers 32, a bottom cover 33, and aflanged inlet duct 34. In the embodiment illustrated, a pyramid reedvalve assembly, generally designated 35, is mounted to each side of themain body 31. The pyramid reed valve assembly 35, as seen in FIGS. 2 and4, is secured, together with a gasket 36, between the rolled-overannular flange 37 of the main body 31 and the end flange of a dischargecasing 32. Each of the discharge casings 32 thus encloses a pyramid reedvalve assembly 35 and, they together with the central main body portion31 form a central inlet chamber 38 and, outboard and on opposite sidesthereof, the discharge chambers 41. Discharge chambers 41 are placed incommunication with air manifold 15 by conduit couplings 17, eachthreadably engaging the externally threaded outlet 42 of a dischargecasing 32.

The inlet chamber 38 is placed in communication with atmospheric air byan aperture 43 in the upper flange 44 of the central main body 31 and bythe flanged inlet duct 34 which is suitably secured to the upper flange44 with a gasket 45 sandwiched therebetween by means of positioningplugs 46 inserted into suitable apertures provided in the external sidewalls of the central main body portion 31. In the arrangement shown inFIG. 1, flanged inlet duct 34 is connected by hose 18 to the air cleaner12, as previously described, a wire hose clamp 47 being used to secureone end of the hose to the flanged inlet duct 34.

Although two pyramid reed valve assemblies 35 are used in the embodimentof the air induction valve 30 disclosed, that is, pyramid reed valveassemblies being mounted on both the lefthand and right-hand pyramidreed valve assembly 35 only the righthand unit is illustrated anddescribed in detail, since each of the pyramid reed valve assemblies isidentical. In the preferred embodiment, each pyramid reed valve assembly35 includes a pyramidal valve seat member 50 for reed valves 51 and athermal reed valve spring retainer 52. The valve seat member 50 is shownas being of hollow frusto-pyramidal form with four inclined side walls53, an end wall 54 and a flanged portion 55 around the base of thepyramid. In each of the side walls 53 there is provided an aperture orport 56, circular in configuration and positioned closely adjacent tothe end wall 54. The portion of each side wall surrounding the apertureor port 56 therein has a smooth outer annular surface lying in a planeabove the extremity of the end wall to provide a valve seat 57 forengagement by a reed valve 51.

Each reed valve 51, formed of a suitable thin flexible material, issecured at one end by rivets 58 to a side wall 53 with its free endpositioned to cover the port 56 with which it cooperates. As shown moreclearly in FIG. 2, the reed valve is perforated between the fixed endand its free end to provide connecting leg portions 59. The fixed end ofeach reed valve is also provided with suitable apertures therein throughwhich the rivets 58 extend to thereby prevent lateral movement of thereed valve with respect to its valve seat.

The free end of the reed valve Sll is ofa length to extend sufficientlyover the farthest extremity of the valve seat 57, as seen in FIG. 4, topermit the outermost free end of the reed valve to overhang this seat.It has been found advantageous to provide for this overhang of theoutermost free end of the reed valve because, during operation, the reedvalve flexes sinusoidally with the free end bending down. Thus, if thereed valve did not overhang its seat sufficiently, the free end of thevalve, which is bent downward during closure, would initially contactthe seat to cause secondary flexing of the reed valve and, as this isrepeated over an extended number of cycle, it will cause damage to thereed valve, such as breaking or splitting of the reed element. With thefree end of the reed valve overhanging the valve seat, as disclosed, thedownward bent free end of the reed valve will overhang the valve seat,and, because of this overhang, it will not engage the valve seat andtherefore, there will be no secondary flexing of the reed valve. Byeliminating this secondary flexing of the reed valve during each closingcycle of the valve, an increase in the useful life of the reed valve iseffected.

In operation, each of the reed valves Sll acts as a check valve topermit air to be drawn through the flanged inlet duct 34 into inletchamber 38 for dis- I charge through the reed valve into dischargechamber 41 for delivery to the injection'tubes 16 by exhaust pressurepulsation, as previously described, while preventing exhaust backflowinto the inlet chamber 38.

In order to assist in the prevention of exhaust backflow, reed valvespring retainer 52 is formed of suitable bimetallic material to act as athermostatic stop for the reed valves and, being moveable with anincrease in temperature, to force the reed valves against their valveseats as the exhaust gases flow back toward the reed valves. In theembodiment shown, the thermal reed valve spring retainer 52, of suitablebimetallic material as previously described, takes the form of an endwall 6H, secured to mounting post 62 on the end wall 54, as by stakingover the free end of the mounting post, with multiple spring fingers 63extending from the end wall 61 over the reed valves with which theycooperate. Thus, the thermal reed valve spring retainer 52 is of an openfrusto-pyramidal from complementary to the valve seat member 50 and actsas a temperature responsive element to hold the reed valves closedagainst their seats during certain adverse operating conditions asdescribed above.

As shown in FIG. 4, each spring finger is normally spaced from itscorresponding reed valve so that the reed valve is free to open.However, with an increase in temperature, the fingers will move in thedirection of the arrow A shown in FIG. 4 whereby the space between thespring finger in the reed valve decreases as the temperature increases.Thus, under high temperature conditions, which exist as exhaust gasesflow back toward the reed valve to cause a sharp radiant tempera tureincrease at the reed valve area, the spring fingers are forced againstthe reed valves as shown in FIG. 2 to push them against their seats thuspreventing backflow of exhaust gases into the inlet chamber 38.

Air flow through flanged inlet duct 34 into the inlet chamber 38 iscontrolled by a valve 711 secured by retainer 72 to the stepped stem 73of valve stem 74, the reduced end of the stem being staked over retainer72. Valve 7ll is adapted to close against valve seat 65 on the base ofthe flanged inlet duct 34, but is normally held open with respect tothis seat by spring 75 which encircles stem 73 with one end of thespring engaging the bottom of the main body 31 and the other endengaging the annular base of valve stem 74.

Movement of the valve 711 from the open position shown in FIG. 4 to theclosed position, not shown, against valve seat 65 is affected by meansof diaphragm 76. The outer periphery of diaphragm 76 is secured againstthe flange of the bottom cover 33 which in turn is held against theunderside of the main body 31 by the rolled over annular flange 66 ofthe main body 31. Diaphragm 76 is provided with a central circularopening and is clamped between the annular base of the valve stem 74 andthe annular outer end of a centrally apertured retainer 77 whichencircles the diaphragm around the opening therein and which is stakedin place by means of pins 78 protruding from the annular base of thevalve stem. The diaphragm 76 thus forms with the bottom cover 33 and thecentrally depressed portion of the main body 31 the chambers 86 and 811.

Communication between the two chambers 80 and 811 is controlled by meansof a timing valve 82 also held in position by the retainer 77 againstthe bottom of the annular base of the valve stem 74. Valve 82, which isa disc of flexible material, is provided with an arcuate slot 83 thereinto form a flapper bleed valve portion which overlies an annular recessin the annular base of the valve stem with the arcuate slot therein incommunication via one or more bleed grooves 84 and a passage 85 in theannular base of the valve stem with the chamber 86. A conduit 66connects chamber 86 via hose conduit 2K to the intake manifold 119 ofthe engine induction system.

During engine deceleration, the high induction vacuum pulls diaphragm 76upwardly against the bias of spring 75 to cause valve 711 to seatagainst its valve seat 65, thus blocking flow of air to the injectiontubes. A seal 87 surrounds the stem 73 to separate inlet chamber 36 fromchamber 60. If this seal were not present, air flow from inlet chamber38 would bleed into chamber 86 and reduce the vacuum causing thediaphragm 76 to flutter and valve 71 to chatter.

As previously described, during engine deceleration, a high vacuumraises diaphragm 76 to close valve 71. At the same time, the timingvalve 32 is closed, but air can bleed between chambers 86 and 811through the bleed groove 84 previously described. After a period,determined by the size of the bleed groove, the pressure in chambers 80and 8H will be sufficiently balanced so that the spring 75 will lowerthe valve 71 out of engagement from the valve seat 65. The size of thebleed groove 64 is selected so that balancing of pressure betweenchambers 80 and 81 will occur over a predetermined time interval so thatvalve 71 is closed only for this time interval.

If, however, before the pressure in chamber 80 is balanced with that inchamber 81, the engine is suddenly accelerated, the pressure in intakemanifold 19, and therefore, chamber 80, will rise rapidly. As thisoccurs, the timing valve 82 will open placing chambers 80 and 81 incommunication with each other so that the pressure in chambers 80 and 81may be quickly balanced with respect to each other so that, in effect,valve 71 is maintained open by spring 75.

What is claimed is:

1. An exhaust emission control system for use on an internal combustionengine having an induction passage and having N number of combustionchambers, said exhaust emission control system comprising exhaust meansincluding manifold means and exhaust pipe means with at least a firstexhaust manifold means and a first exhaust pipe means, said firstexhaust manifold means being connected to not more than four of said Ncombustion chambers, air induction means having air inlet means and airdischarge means, said air discharge means extending into said exhaustmeans to direct air toward said combustion chambers, air induction valvemeans including a housing defining passage means through which air canflow and including inlet means in communication with the atmosphere anddischarge means connected to said air inlet means, check valve means insaid passage means responsive to subambient exhaust pressure in saidexhaust means to control the flow of air to the combustion chambers, avalve means positioned for controlling air flow through said inletmeans, said housing including a chamber, pressure responsive diaphragmmeans dividing said chamber to form first and second pressure chambers,means connecting said valve means to said diaphragm whereby said valvemeans is positioned by said diaphragm, conduit means extending from saidfirst pressure chamber and adapted to be connected to the inductionpassage to subject said first pressure chamber to pressure in theinduction passage, a timing valve including means providing a restrictedpassage for controlling the pressures in said first and second pressurechambers and, means normally biasing said diaphragm means in a valveopening position whereby said valve means is normally maintained in anopen position.

2. An exhaust emission control system according to claim 1 wherein saidexhaust means further includes a second exhaust manifold means andsecond exhaust pipe means, said second exhaust manifold means beingconnected to not more than N/Z number of combustion chambers, said firstexhaust manifold means being connected to the remaining combustionchambers, said air induction valve means including a first set of checkvalve means to supply air to said first exhaust manifold means and asecond set of check valve means to supply air to said second exhaustmanifold means.

3. An exhaust emission control system according to claim 2 wherein saidair induction valve means includes thermal responsive stop meanspositioned adjacent said check valve means to normally allow operationof said check valve means but moveable in response to temperatureincreases to close said check valves means upon sudden increases intemperature adjacent said thermostatic stop means.

4. An air induction valve for use in the exhaust emission control systemof an internal combustion engine having an induction passage and exhaustmanifold means defining a portion of a combustibles flow path andconduit means adapted to supply air to the combustibles flow path, saidair induction valve comprising a housing defining a passage throughwhich air can flow and including an inlet and discharge means, saiddischarge means being adapted for connection to the conduit means, reedvalve seat means positioned in said passage and having therein meansdefining ports and a seat extending from and surrounding each of saidports, a reed member subject to curvature secured at one end to saidseat and having a port closing portion, stop means for each of saidreeds to limit opening of said reeds, valve means positioned forcontrolling air flow through said inlet, said housing further defining achamber pressure responsive diaphragm means dividing said chamber into afirst chamber and a second chamber and being operatively connected forpositioning said valve means and including means to normally bias saidvalve means open relative to said inlet, conduit means extending fromsaid first chamber and adapted to be connected to the induction passageto subject said first chamber to induction pressure, and timing valvemeans including means providing a restricted passage for equalizing thepressures in said first chamber and said second chamber after a periodof time as determined by the size of said restricted passage.

5. An air induction valve according to claim 4 wherein said stop meansis a thermal responsive bimetallic element adapted to hold said reedmembers against their seats when subjected to high temperature exhaustgases.

6. An air induction valve according to claim 4 wherein the free end ofeach of said reed most remote from said end secured to said seat extendsover said seat when said reed is in a port closing position.

7. An air induction valve according to claim 4 wherein two opposed reedvalve seat means are positioned in said passage on opposite sidesrelative to said inlet, each of said reed valve seat means comprises avalve seat member having a truncated pyramidal portion with four sides,an open flanged base and closed end wall, each of said sides havingmeans defining a port, a reed being positioned on each of said sides tooverlie the respective port.

3. An air induction valve according to claim 7 wherein said stop meansare thermal responsive bimetallic elements each of which is of an openfrustopyramidal from complementary to said reed valve seat means havingan end wall secured to said closed end wall and having stop fingersextending over said reeds. t t

1. An exhaust emission control system for use on an internal combustionengine having an induction passage and having N number of combustionchambers, said exhaust emission control system comprising exhaust meansincluding manifold means and exhaust pipe means with at least a firstexhaust manifold means and a first exhaust pipe means, said firstexhaust manifold means being connected to not more than four of said Ncombustion chambers, air induction means having air inlet means and airdischarge means, said air discharge means extending into said exhaustmeans to direct air toward said combustion chambers, air induction valvemeans including a housing defining passage means through which air canflow and including inlet means in communication with the atmosphere anddischarge means connected to said air inlet means, check valve means insaid passage means responsive to sub-ambient exhaust pressure in saidexhaust means to control the flow of air to the combustion chambers, avalve means positioned for controlling air flow through said inletmeans, said housing including a chamber, pressure responsive diaphragmmeans dividing said chamber to form first and second pressure chambers,means connecting said valve means to said diaphragm whereby said valvemeans is positioned by said diaphragm, conduit means extending from saidfirst pressure chamber and adapted to be connected to the inductionpassage to subject said first pressure chamber to pressure in theinduction passage, a timing valve including means prOviding a restrictedpassage for controlling the pressures in said first and second pressurechambers and, means normally biasing said diaphragm means in a valveopening position whereby said valve means is normally maintained in anopen position.
 2. An exhaust emission control system according to claim1 wherein said exhaust means further includes a second exhaust manifoldmeans and second exhaust pipe means, said second exhaust manifold meansbeing connected to not more than N/2 number of combustion chambers, saidfirst exhaust manifold means being connected to the remaining combustionchambers, said air induction valve means including a first set of checkvalve means to supply air to said first exhaust manifold means and asecond set of check valve means to supply air to said second exhaustmanifold means.
 3. An exhaust emission control system according to claim2 wherein said air induction valve means includes thermal responsivestop means positioned adjacent said check valve means to normally allowoperation of said check valve means but moveable in response totemperature increases to close said check valves means upon suddenincreases in temperature adjacent said thermostatic stop means.
 4. Anair induction valve for use in the exhaust emission control system of aninternal combustion engine having an induction passage and exhaustmanifold means defining a portion of a combustibles flow path andconduit means adapted to supply air to the combustibles flow path, saidair induction valve comprising a housing defining a passage throughwhich air can flow and including an inlet and discharge means, saiddischarge means being adapted for connection to the conduit means, reedvalve seat means positioned in said passage and having therein meansdefining ports and a seat extending from and surrounding each of saidports, a reed member subject to curvature secured at one end to saidseat and having a port closing portion, stop means for each of saidreeds to limit opening of said reeds, valve means positioned forcontrolling air flow through said inlet, said housing further defining achamber pressure responsive diaphragm means dividing said chamber into afirst chamber and a second chamber and being operatively connected forpositioning said valve means and including means to normally bias saidvalve means open relative to said inlet, conduit means extending fromsaid first chamber and adapted to be connected to the induction passageto subject said first chamber to induction pressure, and timing valvemeans including means providing a restricted passage for equalizing thepressures in said first chamber and said second chamber after a periodof time as determined by the size of said restricted passage.
 5. An airinduction valve according to claim 4 wherein said stop means is athermal responsive bimetallic element adapted to hold said reed membersagainst their seats when subjected to high temperature exhaust gases. 6.An air induction valve according to claim 4 wherein the free end of eachof said reed most remote from said end secured to said seat extends oversaid seat when said reed is in a port closing position.
 7. An airinduction valve according to claim 4 wherein two opposed reed valve seatmeans are positioned in said passage on opposite sides relative to saidinlet, each of said reed valve seat means comprises a valve seat memberhaving a truncated pyramidal portion with four sides, an open flangedbase and closed end wall, each of said sides having means defining aport, a reed being positioned on each of said sides to overlie therespective port.
 8. An air induction valve according to claim 7 whereinsaid stop means are thermal responsive bimetallic elements each of whichis of an open frusto-pyramidal from complementary to said reed valveseat means having an end wall secured to said closed end wall and havingstop fingers extending over said reeds.